This invention relates generally to fluid pumps. More particularly it relates to a gear pump having provisions for maintaining pressure distribution at acceptable levels when the pump is operated in an environment wherein conditions beyond original design specifications are encountered.
During operation of a gear pump, the gear teeth rotate in a gear pocket bore. As the teeth move into contact with the pump housing at the inlet seal point, pockets are formed in which fluid is trapped. With continued rotation of the gears, fluid pressure rises as these pockets move toward the pump outlet. When the gear teeth reach what may be termed the discharge seal point, the pockets are communicated with the pump outlet. Thus there are three zones, a first, inlet zone at inlet pressure, a second, intermediate zone between the inlet and outlet seal points in which there is a pressure rise from inlet pressure to discharge pressure, and a third, outlet zone at discharge pressure. The internal pressure distribution profile between the seal points is a matter of pump design.
When a gear pump is operated under load, volumetric efficiency is maintained by loading at least one pressure plate in contact with the side faces of the gears to close pump clearance gaps. During the pumping operation, fluid pressure increases as it is moved from the inlet to the outlet while trapped in the pockets. Since a fluid under pressure creates a force acting equally in all directions, a force exists which acts in a direction parallel to the axes of the gears. This force tends to bias the pressure plate away from the gears. To counteract this, a fixed area on the back of housing side of the plate is sealed to contain a compartment of oil at discharge pressure. Since this area is fixed, the force created here is constant for any given discharge pressure. Optimum pump design does not balance the force on the front or gear side of the plate, but rather overbalances slightly so as to bias the plate toward the gears in such a manner that a high degree of volumetric efficiency is maintained. However, the magnitude of the force on the front side of the plate is dependent upon a number of variables including discharge pressure, pump speed, fluid viscosity and inlet pressure, while the magnitude of the force on the back side of the plate is dependent only upon discharge pressure. Thus, the degree of overbalance varies considerably from one set of operating conditions to another.
Heretofore a typical pump design might anticipate that the inlet is subjected to atmospheric pressure or a slight sub-atmospheric pressure such as, for example, a vaccum of five inches of mercury. When such a pump is subjected to a vacuum of, for example, fifteen inches of mercury at the inlet, the pressure distribution profile is distorted such that the net force biasing the pressure plate toward the gears is increased. This results in a reduction of pump efficiency. Although the force on the back side of the plate could be reduced, such reduction would require additional tooling and modified seals.
There remains a need in the art for a gear pump which provides a simple, efficient and economical way to compensate for the overbalance which is encountered when the pump is operated beyond design applications with a corresponding decrease in pump efficiency. One particular application would be, for example, where the pump inlet is subjected to a higher vacuum than would have been considered practical heretofore.